The invention relates to a tool for automatically forming, on thermoplastic pipes, bellings of considerable longitudinal extension for sealingly joining pipes subject to high axial stresses. It comprises a spindle on which there is axially superimposed, by means of sliding, the heated end of a thermoplastic pipe which is deformed plastically in the region of a shaped portion of the spindle designed to define an annular seat for a sealing gasket of the pipe itself.
Tools are known for forming bellings on pipes made of thermoplastic material which comprise an internal annular seat designed to receive a ring or sealing gasket in the joint between two consecutive pipes, as well as a flaring or bell, located downstream of the said annular seat, designed to receive the end of the pipe connected to the pipe provided with the belling as described.
Within the range of such tools there are those which comprise a pressurized-air chamber which acts from the outside of the pipe so as to form both the annular seat and the bell, or so as to form the entire belling. In this case the spindle is provided with a shaped portion in relief, consisting of a ring of segments which can be expanded and retracted radially and which are designed to form the internal annular seat for the seal of the thermoplastic pipe which, at its front end, abuts against a heading flange surrounding there the spindle itself.
The spindle also has, downstream of the aforementioned shaped portion, a radial flaring which is designed to form the base of the belling and connect it to the remainder of the pipe, which is of nominal diameter.
In these cases therefore, the preheated end of the pipe must first be expanded radially along the radial flaring of the spindle and must then be expanded more extensively in the region of the expandable shaped portion. The latter is then retracted allowing removal of the pipe formed with its complete belling, obviously following cooling of the thermoformed part.
These bellings are particularly precise and have minimum tolerances even along the annular seat, even though the movement of mechanical parts occurs there, since this tool involves the action of a pressure which acts from the outside of the pipe towards a precise internal shaped portion, thereby ensuring perfect calibration internally. This obviously results in equally precise as well as stable and efficient positioning of the sealing gasket or ring of the belling.
However, there exists a significant drawback, due to the risk of the preheated pipe collapsing in the region of the zone where it undergoes the preliminary expansion, during its insertion onto the spindle. This risk of collapse is all the greater the longer the belling which one wishes to obtain. Consequently, there are precise limits as regards the length, which must be respected if damage to the belling is to be avoided.
In another known tool, on the other hand, a pressurized air chamber is provided, which acts from the inside, instead of from the outside of the pipe, so as to form the entire belling, including the annular seat. In this case there is provided a spindle which is shaped with a slight flaring so as to form the base of the belling and determine the diameter of the end of the belling itself. The spindle is also not provided with the expandable shaped portion which, in the preceding tool, was used to form the annular seat of the sealing ring. In this second tool the belling is in fact determined by the expansion of the preheated pipe section, against an external mold, against which the pressurized fluid acts. This outward expansion also takes place in order to form the annular seat for the seal. Basically, when the preheated pipe section is inserted onto the spindle during the initial phase, it does not encounter any particular obstacles or opposing forces while it is superimposed, by means of sliding, onto a spindle of this kind. Therefore, there is no risk of the pipe collapsing in this case. However, there is the obvious drawback due to the fact that the precision and the internal calibration of the belling is certainly not exceptional in such a case, since the mechanical abutment on the belling is located on the outside of the pipe and not, as in the preceding tool, inside the said pipe.
Therefore, with this second tool, it certainly would be possible to provide a belling which may be of any length, but not calibrated in a sufficiently precise manner as to guarantee subsequently the sealing action between successive pipes, especially if these joints were to be laid in sites which are subject to telluric movements or in any case are subject to intense axial deformations or stresses.